1. Field of the Invention
This invention relates to a liquid ejection head for ejecting liquid such as ink toward various types of printing media such as a sheet of paper.
2. Description of the Related Art
Currently, the typically employed printing methods of ejecting liquid such as ink include an ink jet printing method. The ink jet printing method employs an electrothermal conversion element (heater) or a piezoelectric element as an ejecting energy generating element to eject liquid. In the use of either element, the liquid can be controlled by an electric signal.
In recent years, a reduction in size of droplets ejected and an increase in the number of nozzles in the liquid ejecting head have been developed in response to a growing need for increasing the image quality of printing. Along with this, an increasingly serious matter is the effects on printing of droplets not contributing to the printing, in addition to droplets ejected for printing. Specifically, upon the ejection of the liquid, the stream of liquid breaks up to form the main droplets and the sub droplets (hereinafter referred to as “satellite droplets”). The main droplets land on the desired location of the printing medium, whereas the landing location of the satellite droplets may possibly not be controlled. In the case of conventional low image-quality printing, the effects of the satellite droplets on print are almost negligible. However, with an increase in high image-quality printing, the reduction in image quality caused by the satellite droplets becomes increasingly obvious. In addition, small-sized satellite droplets lose their velocity before reaching the printing medium to form ink drops floating in the air (hereinafter referred to as “mist”). The mist may stain the printing apparatus. In turn, the stain in the printing apparatus may be transferred to the printing medium to stain the printing medium.
As a method for preventing the satellite droplet formation, Japanese Patent Laid-Open No. H10-235874 discloses a method of providing an ejection orifice formed in a shape other than a circle in order to reduce the number of satellite droplets. In the method disclosed in Japanese Patent Laid-Open No. H10-235874, the ejection orifice has a long periphery because it has a shape other than a circular shape.
In liquid ejection from a conventional ink jet print head, when the nozzle is re-operated for printing after a rest over a fixed time period, the first ink drop may possibly not be ejected or alternatively may possibly, without traveling straight, land on an unintended place in the printing medium. Causes of such uneven liquid ejection after the lapse of a fixed time period include an increase in ink viscosity because of the evaporation of the ink in the nozzle during the printing rest.
One of the factors in uneven ejection after a lapse of a predetermined time period involves a flow resistance at the ejection orifice and the like. That is, a high flow resistance results in uneven ink ejection. As a result, the ink cannot be smoothly ejected after the lapse of a predetermined time period.
When an ejection orifice has a long periphery as disclosed in Japanese Patent Laid-Open No. H10-235874, the flow resistance increases during ejection. For the purpose of reducing the number of satellite droplets, the provision of a protrusion in the ejection orifice to increase the periphery of the orifice is effective. However, the protrusion causes an increase in flow resistance. The provision of the protrusion may hinder the ejection smoothness after the lapse of a predetermined time period. In other words, a reduction in the number of satellite droplets and the improvement of the ejection smoothness after the lapse of a predetermined time period counteract each other. However, an important element for the achievement of high grade print is to improve the ejection smoothness after the lapse of a predetermined time period while the number of satellite droplets is reduced by use of a non-circular shaped ejection orifice.
A method for improving the ejection smoothness after the lapse of a predetermined time period is disclosed in, for example, Japanese Patent Laid-Open No. 2004-209741 which discloses a method of preventing the ejection from deteriorating after the lapse of a predetermined time period in which holes (moisture retention holes) of a size not allowing ink to be ejected are provided around an ejection orifice, in order for the ink to be evaporated from these holes, so that the moisture around the ejection orifice is maintained.
Japanese Patent Laid-Open No. 2004-209741 discloses a structure having moisture retention holes of 3 μm to 4 μm in diameter arranged around the ejection orifice. Because of the very small diameter of each moisture retention hole itself, the ink is apt to solidify in the moisture retention holes during the time when the printing operation is not being performed. Even if a sucking recovery operation is performed for preventing the ink from solidifying, since the resistance is smaller in the ejection orifice than in the moisture retention holes, which are smaller in diameter than the ejection orifice, the ink is sucked from the ejection orifice. This makes it difficult to remove the ink solidifying in the moisture retention holes. Thus, the provision of the moisture retention holes fall short of reducing the amount of ink evaporated from the ejection orifice. In view of the various environments in which the liquid ejection head is mounted, the moisturizing measures to improve the ejection smoothness after the lapse of a predetermined time period fail to deal with many situations.
Particularly, such defective conditions deteriorating smooth ink-ejection after the lapse of a predetermined time period easily occur in the area close to the end of a nozzle row. For this reason, nozzle misfiring at the nozzle ends or droplet misdirection (deflection in the ejected direction) may possibly reduce the print quality.